京都府京丹後市河辺白石山産モナズ石の化学組成とCHIME年代

抄録

Monazite crystals from a pegmatite within the Miyazu Granite, located at Shiraishiyama, Kobe, Kyotango, Kyoto Prefecture, Japan, were analyzed for U, Th, Pb, and rare earth element (REE) concentrations using electron microprobe analysis. Back-scattered electron imaging revealed both oscillatory and sector zoning within the monazite. The crystals exhibit a solid solution between monazite and huttonite components, with a maximum ThO₂ content of 30.81 wt%. The empirical formula of the Th-richest composition is (Ce_0.336La_0.151Nd_0.098Y_0.041Pr_0.033Sm_0.016Gd_0.008Dy_0.004Er_0.002Th_0.309U_0.009Ca_0.003)_Σ1.010 (P_0.660Si_0.338)_Σ0.998O₄. The observed compositional variations suggest that the (Th + U) content is primarily controlled by the coupled substitution (Th, U) + Si = REE + P. The CHIME age of the monazite grains is 61 ± 7 Ma (2σ), which is consistent with previously reported radiometric ages of the Miyazu Granite.